1. Field of the Invention
The present invention relates to an image reading device which reads a color image recorded in a negative film by using an optical sensor.
2. Description of the Related Art
Conventionally, in the image reading device, an exposure measurement is performed, prior to a pre-scanning operation and a regular scanning operation, by which image data, which will be output to a personal computer, are sensed, so that an optimum exposure time, for which electric charges are accumulated in a line sensor in the pre-scanning operation and the regular scanning operation, is calculated. In the exposure measurement, the image is scanned with a pitch coarser than that of the regular scanning operation, and the image data are sensed by an operation in which the line sensor is exposed with a relatively short exposure time. The optimum exposure time is obtained by analyzing the image data. The analysis of the image data is carried out based on a histogram indicating a distribution of pixel values contained in the image data.
Then, a scanning operation is performed again with a coarser pitch, using the optimum exposure time, so that exposure parameters are calculated based on the image data, to carry out a color correction of the color image which is to be indicated by a display device of the personal computer. The exposure parameters are obtained based on a histogram indicating a distribution of pixel values of the image data, similarly to the calculation of the optimum exposure time.
The dynamic range of negative film is broad, and therefore, densities of the recorded film are broadly scattered. Further, output characteristics of the line sensor are non-linear, and thus, the tendency for non-linearity is great especially when the output signal is low. Therefore, if the image data are always sensed with a constant exposure time in the exposure measurement, a problem occurs in which the accuracy of the sensing of exposure parameters is low, because of the non-linearity of the output characteristics of the line sensor, when an object image is dark such that the film density is high.
Therefore, an object of the present invention is to provide an exposure control device which always obtains the exposure parameters with a high accuracy and in a short time, irrespective of the film density.
According to the present invention, there is provided an exposure control device provided in an image reading device which reads a color image recorded in a negative film, the exposure control device comprising an image reading processor, a histogram generating processor, a histogram selecting processor, and an optimum exposure time calculation processor.
The image reading processor exposes an optical sensor to obtain image data corresponding to the color image, using more than two kinds of exposure times, which are different from each other. The image data are generated regarding the exposure times, respectively. The histogram generating processor generates histograms showing distributions of pixel values of the image data, regarding each of the exposure times. The histogram selecting processor obtains a maximum effective value in each of the histograms, and selects a first histogram, in which the maximum effective value is less than or equal to a predetermined value and is the greatest value among the histograms, and a second histogram corresponding to the image data obtained by using one of the exposure times which is subsequently shorter than the exposure time of the first histogram. The optimum exposure time calculation processor calculates an optimum exposure time based on the maximum effective values of the first and second histograms.